Android Phone Based Metal Detector - FCMD

[moodz]

Below is the data from my transmit circuit where the supply voltage is 12 volts and the transmit frequency close to 30 Khz.
The circuit is drawing 89 ma from the power supply but the generated current in the transmit coil is nearly +/- 1.8 amps with an inductance of 0.5 mH and Rcoil = 0.5 ohms.
= 1W = ok


It runs on 1.2 ( one point two ) volts. The power consumption is 1 watt. = 0.8A x 1.2V = ?

I am using one of those common chinese switchmode supplies ... it runs of 12 volts at the moment but this project will probably be running off a 5 volt source eventually.
The minimum voltage from the o/p of the supply is 1.2 volts ... I measured the current and it was 840 mA @ 1.2 volts = 1.008 watts.
The switch mode is running around 96% efficient ... so I am pretty happy.

If I run the E class in its other ( super resonant ) mode then I get 400 ma peaks in the coil but the supply only draws 10 ma from 12 volts.

[pito]

Below is the data from my transmit circuit where the supply voltage is 12 volts and the transmit frequency close to 30 Khz.
The circuit is drawing 89 ma from the power supply but the generated current in the transmit coil is nearly +/- 1.8 amps with an inductance of 0.5 mH and Rcoil = 0.5 ohms.
= 1W = ok


It runs on 1.2 ( one point two ) volts. The power consumption is 1 watt. = 0.8A x 1.2V = ?

[moodz]

Built the E class amplifier / driver ... only uses 5 jellybean parts.

Below is the output waveform across the TX coil. 150 volts peak to peak. @ 32Khz.
The coil current is approx 2.4 amps peak to peak.

It runs on 1.2 ( one point two ) volts. The power consumption is 1 watt.
The efficiency is very high.
Its so efficient if you lift the supply voltage the growth of current in the coil is exponential .... and it would be like having an induction cooker on a stick instead of a metal detector.


The spectrum ( pink trace ) is clean over DC to 1.25 Mhz.
Now I can start work on the low noise amplifier front end.

[boilcoil]

OK.

[moodz]

In practice, that's not exactly the case.

Abstract
"The Class E power amplifier has a big popularity because of its high efficiency. The above can be achieved in optimum class E when both soft commutation conditions have been met– zero voltage and zero voltage slope of during turn on. Meeting only the one of the above conditions is described as sub-optimum mode".

The graph below is for three modes:
- optimal s = 0
- suboptimal s < 0
- suboptimal s > 0



The actually achieved efficiency coefficient is almost 95% in optimal mode.

... yes that might be the case for resistive loads since most class E amps are used in RF applications with 50 or 75 ohms etc however when the class E amp was originally developed in the 70s one of the inventors aims was to drive inductive loads ( eg antennas with high inductance ) efficiently.
Similiarly the metal detector transmit coil is mostly inductance with relatively small resistance.
Below is the data from my transmit circuit where the supply voltage is 12 volts and the transmit frequency close to 30 Khz.
The circuit is drawing 89 ma from the power supply but the generated current in the transmit coil is nearly +/- 1.8 amps with an inductance of 0.5 mH and Rcoil = 0.5 ohms.
There is no resonating capacitor across the transmit coil ( except for parasitic of about 300 pF ) so the self resonance of the coil is far away from the driving frequency of the E class amp. The red voltage spikes are at the drain of the mosfet. The dark blue trace is the gate drive. The light blue trace is the current drawn from the supply and the green trace is the transmit coil current.

The top panel traces clearly show the mosfet is operating in ZVS mode and locked to the transmit pulses from the CPU.

The efficiency with this circuit is not really the primary concern ....but it is certainly excellent for a battery operated detector ( eg lithium ).

At 89 ma from 12 volts it is drawing around 1 watt from the supply. A 3 cell 18650 pack would run this transmitter for about 10 hours.

[boilcoil]

In practice, that's not exactly the case.

Abstract
"The Class E power amplifier has a big popularity because of its high efficiency. The above can be achieved in optimum class E when both soft commutation conditions have been met– zero voltage and zero voltage slope of during turn on. Meeting only the one of the above conditions is described as sub-optimum mode".

The graph below is for three modes:
- optimal s = 0
- suboptimal s < 0
- suboptimal s > 0



The actually achieved efficiency coefficient is almost 95% in optimal mode.

[moodz]

It would be smarter if you could work in the joint/common region of ZVS and ZSS for maximum efficiency.

is 99.4 % efficiency not high enough ???

E class does use ZVS... not sure where ZSS comes in thats more of a motor control parameter.

[boilcoil]

Something is a duplicate post.

[boilcoil]

It would be smarter if you could work in the joint/common region of ZVS and ZSS for maximum efficiency.

[moodz]

I have been using the bipolar pulsing H bridge which uses 6 mosfets + driver chip ..... the bipolar square waves are nice and wideband but produce transitions that cause problems in the RX chain.

So I am switching ( there is a small pun there ) to an E class amplifier to generate the TX signal.
This amp only uses a single mosfet and some passive components.
It was invented back in the 70s.

The secret sauce is in the component values and the timing ... especially the timing
Even though the circuit is simple ... there is alot going on in there.
LT is the transmit coil.

There is also an F class amplifier ... that is for harmonics ... Its not suitable for the FCMD so I did not use it.

Long story short I got the AI to build me a fully auto pushbutton tool that generates the correct values for the E class amplifier and now I am looking at very nice high power efficient transmit signals.
It might look like the components have to be spot on ... but that is not the case. When you find the operating point using the tool the amplifer can work over a range of timings / frequencies.

Below is the tool ... I am still modding it so no point releasing anything yet.
It also builds an ltspice circuit once you find your optimal specs so the simulation can be checked.

[Aziz]

...hmm well you should know there is no physical system to achieve EMI cancellation using external coils or fairy dust within a 1 hz bandwidth.
The other problem is a that with a 1hz bandwidth the detector would be unusable due to lag.
I use 50hz as the token ouput rate on the FCMD detector ... so the bandwith is 25 hz.
The noise floor is around -110db to -120db ... the noise you do measure there is coming from the ADC reference ... not EMI.
You cant fix that with a coil ... it needs better components.
... also remember that EMI is not synchronous ... synchronous EMI can only come from your own system. No coils involved.
If you are picking up non synchrounous EMI then your code is broken.

My biggest noise source is the sound card (DAC, output amplifier, ADC, input amplifier, ADC reference, power supply, etc.. ).
So I have ignited the next stage to overcome the issues. Using an amplifier. This should improve the overall SNR by 20 - 30 dB (depending on the gain).
I hope to get with the mono coil VLF close to the performance of a DD coil configuration.

The DD coil configuration delivers similar detection depth performance like yours. But this is trivial.
I want to have the mono coil VLF. Right now. Immediately.
Cheers

[moodz]

Nice Paul.

But my sound card is adding noise. And yes, the EMI noise is also present. With Q-factor beyond 100 I am working with with mono coil VLF, there is EMI noise. We can't ignore it. Even if my demodulator bandwidth is very very low (< 1 Hz).

This isn't an issue, if we use standard IB coil configuration with higher TX current however. This is very easy to get there.
But I am still working on mono coil VLF design.
Aziz

...hmm well you should know there is no physical system to achieve EMI cancellation using external coils or fairy dust within a 1 hz bandwidth.
The other problem is a that with a 1hz bandwidth the detector would be unusable due to lag.
I use 50hz as the token ouput rate on the FCMD detector ... so the bandwith is 25 hz.
The noise floor is around -110db to -120db ... the noise you do measure there is coming from the ADC reference ... not EMI.
You cant fix that with a coil ... it needs better components.
... also remember that EMI is not synchronous ... synchronous EMI can only come from your own system. No coils involved.
If you are picking up non synchrounous EMI then your code is broken.

[Aziz]

Nice Paul.

But my sound card is adding noise. And yes, the EMI noise is also present. With Q-factor beyond 100 I am working with with mono coil VLF, there is EMI noise. We can't ignore it. Even if my demodulator bandwidth is very very low (< 1 Hz).

This isn't an issue, if we use standard IB coil configuration with higher TX current however. This is very easy to get there.
But I am still working on mono coil VLF design.
Aziz

[moodz]

..so we have achieved good sensitivity for target detection in FCMD.
We are now getting the corresponding accuracy in discrimination ( without phase rotations or reference phases I might add ! )

The phase accuracy is not impacted by GB either. ( though amplitude will drop a bit for some targets aligned in the phase plane with the ground. )

Below we have copper, silver, gold, al foil, ferrite.
The orange ring is the threshold setting ... anything exceed the threshold gets a phase tag.

Theres no EMI ... because EMI is not a big issue in narrowband systems ( Aziz )

[moodz]

Yes, I also use these STM32H7 boards and they are really great, the STM32WB board is good for wireless applications for transmission via Bluetooth or the LLD method which works similarly to the NRF24L01, the only disadvantage of these chips is that they do not have a 16bit ADC like the STM32H7 has.

That is the point I was trying to make.
Yes there are boards that have great bluetooth or whatever ... .but are you trying to build a metal detector or a great wireless box ?

A good and cost effective method of capturing the target signal ( ie ADC in this case ) ...is way higher priority in the design criteria than a wireless system.